The following abbreviations are herewith defined.
ADCanalog-to-digital converterAMamplitude modulationASICapplication specific integrated circuitBBbasebandCDMAcode division multiple accessCMRRcommon-mode-rejection ratioCPUcentral processing unitDS-CDMAdirect sequence CDMADSPdigital signal processingFDDfrequency division duplexingFMfrequency modulationFPGAfield programmable gate arrayICintegrated circuitICPinput compression pointIFintermediate frequencyIIP2second-order input intercept pointIIP3third-order input intercept pointIMD2second-order intermodulation productIMD3third-order intermodulation productLNAlow noise amplifierLOlocal oscillatorPMphase modulationPDphase detectorPDFphase-frequency detectorRXreceiverRFradio frequencyRSSIreceived signal strength indicatorTXtransmitterVCOvoltage controlled oscillatorWCDMAwide-band CDMA3Gthird-generation (cellular system)
As is well known, passive components that are used in RF ICs typically have relatively large process variations. This leads to a direct trade-off between the accuracy of the resonant or resonance frequency and the bandwidth of the circuit. As a result it is common practice to use relatively low-Q resonators in the RF signal path in order to ensure a sufficiently wide bandwidth and, thus, sufficient performance without requiring calibration. Additionally, calibrations performed during fabrication are preferably avoided in order to reduce cost. The use of a narrow bandwidth (narrow band) LNA in the RF receiver enables the elimination of a bandpass filter after the LNA, and thus reduces cost. Since the passive component process variations can be large, however, some calibration is normally needed, and the cost savings may not be as great as one would at first expect.
As such, what is required is a simple implementation of a calibration technique that can be used to tune resonators in analog circuits, as well as a technique to utilize (relatively) narrow-band resonators in a radio system.
In most applications a relatively wide-band LNA is used, which is insensitive to process variations, and if necessary an external filter is placed between the LNA and a downstream mixer in order to reduce transmitter leakage (an undesired signal coupled into the receiver from the transmitter). In addition, some structures that use additional resonators in the LNA-mixer interface, or in the LNA topology itself have been presented. Reference in this regard can be made to J. A. Macedo, M. A. Copeland, “A 1.9-GHz Silicon Receiver with Monolithic Image Filtering”, IEEE J. Solid-State Circuits, vol. 33, pp. 378–386, March 1998, as well as to H. Samavati, H. R. Rategh, T. H. Lee, “A 5-GHz CMOS Wireless LAN Receiver Front End”, IEEE J. Solid-State Circuits, vol.35, pp.765–772, March 2000. While primarily intended for image rejection purposes, the problems associated with filtering out-of-band signal components are basically the same as when filtering transmitter leakage. However, although the LNA structure with two resonators has been described in the prior art, an adequate solution to the calibration and optimal scaling with current of the two resonator LNA has not previously been proposed.
FIG. 1 shows a conventional direct conversion receiver 1. After the antenna 2 the desired radio band (e.g., WCDMA/GSM/or other) is selected using a bandpass filter 3 in front of the first (variable) amplifying stage 4. The signal is then downconverted with mixers 5 to a zero IF (i.e. direct conversion) using quadrature local oscillator (LO) signals 6 (in 90 degree phase shift) that are tuned with a synthesizer 7 at the carrier frequency of the received channel. After downconversion the signal is applied to baseband amplifiers 8, 10 and filters 9, and in a digital communications systems the information is converted into digital form with an analog-to-digital converter (A/D) 11 and further then digitally filtered 12. Channel decoding 13 and other necessary digital functions to recover the transmitted information are be performed after the A/D 11. Gain control is an important function to extend the input signal range in all receiver architectures, and is used as well in the instant invention to adjust signal levels during calibration. An RSSI block 14 provides a signal to a gain control logic block 15 that functions to adjust the gain of the amplifiers 4, 8 and 10 to maintain the received signal at a desired level.
FIG. 2A shows the construction of a typical low noise amplifier (LNA) that is used as the first amplifier 4 in the receiver chain, while FIG. 2B shows a typical voltage controlled oscillator (VCO) 7A used in the synthesizer 7. The VCO 7A generates the high-frequency signal from which the quadrature LO signals are generated. Both of these devices use a resonator that can be implemented on the RF IC or with an external tank circuit. Although the resonators in FIGS. 2A and 2B appear slightly different, they perform electrically exactly the same resonance function. Generally, for the purposes of this invention all described resonators can be considered to be an LC tank circuit containing an inductor, a capacitor and a resistor. The resistor is not necessarily shown in all drawings, and in most cases the resistor is actually the parasitic resistance, which degrades the quality of the tank circuit in all physical realizations, and hence must be taken into account.
However, a wide-band LNA can be implemented according to FIG. 2A using a high-quality LC tank and a parallel separate resistor R to enhance the bandwidth by lowering the quality factor (Q value) of the tank circuit. This technique is generally used in many LNA implementations, as the current IC technologies provide inductors and capacitors that produce too high a Q value for the tank circuit, if process variations are taken into account. This problem is illustrated in FIG. 3A. Due to process variations the center frequency can vary by too large an amount between samples in order to cover the entire band of interest (i.e., bandwidth of the system) without requiring tuning (calibration). Therefore, tuning is required in the case of a narrow-band LNA. In the case of a wide-band structure (shown by the dashed lines), the process variations have a much smaller effect on the amplification in the band of interest. The difference between the two approaches in attenuating the out-of-band interferer, such as the transmitter leakage, is shown in FIG. 3B. The benefit of using a narrow-band LNA is obvious, and even relatively small improvements in the attenuation can relax the receiver specifications significantly.
In making system calculations it can be shown that 6 dB attenuation in the maximum transmitter (TX) power leakage at some distance (in MHz) from the desired signal can relax the mixer 5 specification sufficiently so as to remove a filter from the LNA-mixer interface. The intermodulation of the TX leakage with an unwanted spurious signal is considered in this estimation. While a resonance circuit with sufficient performance can be implemented using current IC technologies, the accuracy of the resonant frequency is not acceptable without tuning. However, and as was described above, the requirement to provide tuning increases the cost, and is thus not desirable.